Passive microwave direction finding with monobit fourier transformation receiver and matrix coupled antenna

ABSTRACT

A passive instantaneous microwave signal frequency identifying and angle of arrival identifying system employing a beam steering multiple element antenna coupled through a Butler matrix to a plurality of simplified Fourier transformation radio receivers wherein unit value or near unit value magnitudes of the Kernel function                 -   j2                   π                 kn     N                     
     in the receivers&#39; Fourier transformations are used to elude transformation computation complexity and resulting radio receiver size and cost penalties. Limiting amplifiers, a video signal path and direction finding encoding logic are also employed. Airborne military application of the system in electronic warfare direction finding, radar frequency determination and other activities is included.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalty.

BACKGROUND OF THE INVENTION

When a military aircraft flying in unknown or hostile air space isdiscovered by a distant radar apparatus it is often beneficial for thecrew of the aircraft to not only be made aware of the occurrence of thisradar discovery but to also be appraised of as much informationregarding the discovering radar as is possible. Two significant portionsof this discovering radar information are the physical location of andthe operating frequency of the distant radar apparatus. In addition tothe fundamental act of receiving such information concerning thediscovering radar apparatus it is desirable that this information becomeavailable to the aircraft crew as quickly as is possible and that theinformation be obtained from as little as one pulse of energy receivedfrom a threat signal source. The obtaining of this and other informationsuch as pulse duration and signal strength data in a passive non-signalradiating manner from a distant threat signal is the role of theelectronic warfare radio receiver.

The radio receiver arrangement we have identified by the name of a“monobit receiver” offers an attractive basis for fabricating such anelectronic warfare receiver and for solving several problems arising inthe electronic warfare and other military electronics fields ofendeavor. One group of such problems is locating the source of a distantradio frequency emission from a single pulse of received radio frequencyenergy emission i.e., providing a radio frequency direction findingcapability that is usable in the present day passive monopulseelectronic signal environment. Although radio receivers technicallycapable of performing in this direction finding and frequencyidentification environment have existed for some time the cost,technical complexity and physical size of each such existing receiversand related problems such as relatively short intervals of mean timebetween receiver failure events have limited the practicality ofdirection finding apparatus using existing electronic warfare receivers.This limitation is especially notable with respect to locating suchapparatus within the confines of and within the weight limitations of ahost aircraft such as a tactical or fighter aircraft.

It has been clear to persons working in the monopulse systems technicalfield that a passive instantaneous direction finding apparatus builtaround a multiple element directive antenna having elements coupledthrough a phase responsive network such as a Butler matrix to aplurality of individual radio receivers would be within the realm oftechnical possibility except for the penalty of cost, technicalcomplexity and physical size associated with each of the individualradio receivers needed to embody such a system. Indeed persons workingin this field have proposed such direction finding and frequencyidentification systems in considerable detail. One such system is forexample disclosed in the 1996 U.S. Pat. No. 5,568,154 of Yakov Cohen ofHaifa, Israel. The Cohen '154 patent indeed involves a frequency anddirection finding system inclusive of a multiple element directionalantenna, a Butler matrix and radio receivers assembled into acombination providing first blush similarity to the system of thepresent invention.

A more detailed consideration of the Cohen direction finding andfrequency identification system reveals however the use of several radioreceivers of one of the types identified as “channelized receivers,Bragg cell receivers, compressive receivers (and) digital FFTreceivers”, see column 1, line 19 of the Cohen patent. Five of aselected one of such receiver types are included at 122-130 of the Cohenpatent's exemplary FIG. 1 direction finding system drawing. When thecost, technical complexity and physical size associated with each ofthese previous receiver types is considered, the limited utility of theresulting Cohen FIG. 1 system, particularly in a small aircraft, beginsto emerge however. The inventors of the present invention have usedreceivers of these previous types in experimental laboratory work and infact one of the present inventors has authored a published text book inwhich both technical characteristics and physical embodiment photographsof individual receivers of this type appear. See the Text “MicrowaveReceivers With Electronic Warfare Applications” authored by JamesBao-Yen Tsui, published by John Wiley and Sons, copyright 1986.Photographs of circa mid 1980's versions of receivers of thesechannelized receivers, Bragg cell receivers, compressive receivers anddigital FFT receiver types appear on pages 229, 330, 279 and 183respectively in the Tsui text. From these photographs the physicalsizeportion of the difficulties attending a system according to the Cohenpatent, using five or more of such receivers, becomes apparent. In theinterest of simplifying and shortening the present document neverthelessthe contents of both the Cohen patent and the Tsui text are herebyincorporated by reference herein. At the very least these documentsprovide enlightening background and signal characteristics information.Another text providing helpful background information with respect tothe present invention is the text “Microwave Passive Direction Finding”authored by Stephen E. Lipsky, also published by John Wiley and Sons,and copyright 1987. The Lipsky text is also hereby incorporated byreference herein.

A significant part of the difficulty with the previous digital FFTreceivers heretofore potentially used in monopulse frequency anddirection-finding applications relates to the algorithm used to embodythe Fourier transformation operation in the receiver. Most Fouriertransformation realizations necessarily include an extensive use ofnumeric multiplication in computing values related to the kernelfunction portion, $^{\frac{{- {j2}}\quad \pi \quad {kn}}{N}},$

i.e., the exponential of “e” the base of the natural logarithm, withinthe Fourier transformation algorithm. Both the number of and the size ofeach individual of these multiplications contributes to the complexityof rigorously implementing the Fourier transformation in either hardwareor software form and especially to the difficulty of implementing thisoperation in real time. In an effort to reduce this complexity one ofthe present invention inventors, James B.Y. Tsui and a number ofcolleagues, have shown that Fourier transformation Kernel functions ofunit magnitude or substantially unit magnitude may be used tosuccessfully approximate a true Kernel function value and enable therealization of a Fourier transformation using only multiplication byunity or in essence no multiplication in the Fourier transformationcomputation algorithm. Kernel function realization in this manner isdisclosed in a first U.S. Patent of Tsui et al., a patent numbered U.S.Pat. No. 5,917,737, wherein Kernel function values are located on acircle of unit radius at angular locations of π/4, 3π/4, 5π/4 and 7π/4radians.

A later patent document involving inventor Tsui and colleagues whereinthe Kernel function values are moved on the circle of unit radius tolocations of 0, π2, π, and 3π/2 radians is identified as U.S. Pat. No.5,963,164. In a yet later patent document, the U.S. patent applicationidentified with Ser. No. 09/944,616 and filed on Sep. 4, 2001, inventorTsui and a colleague have demonstrated advantages available when Kernelfunction values located at each of the π/4, 3π/4, 5π/4 and 7π/4 radianlocations are added to the Kernel function values at 0, π/2, π, and 3π/2radians with the added four values being slightly increased in magnitudefrom unit circle values and in fact having a magnitude of (2)^(1/2) or1.414.

The incentive for improving the Kernel function approximations over thatof the earlier U.S. Pat. No. 5,917,737 patent is ease of realizing theapproximation in the transition of the U.S. Pat. No. 5,917,737 patent tothat of the U.S. Pat. No. 5,963,164 patent and a desire for improvedreceiver signal amplitude tolerance or dynamic range responseenhancement in the serial number instance. The FIG. 4 drawing hereinshows the eight unit value-related Kernel function approximationlocations disclosed in the Ser. No. 09/944,616, Sep. 4, 2001 applicationin graphic form and also demonstrates Kernel function locations usablein the present invention. These same eight Kernel function locations arealso used in the invention of U.S. patent application of Ser. No.10/008,476, applicants' attorneys docket number AFD 481, filed in Dec.2001.

Notwithstanding the previous attribute of having a somewhat limited twotone dynamic range characteristic the monobit receiver using limitedKernel function values is nevertheless believed an attractivearrangement for use in a passive microwave frequency direction findingsystem. Additional improvements and performance enhancements currentlyunder investigation for this receiver suggest the possibility of evengreater attraction to the monobit receiver configuration for directionfinding use. The relatively low cost, small physical size and simplicityof any version of this monobit receiver are especially seen as welcomeadditions to the currently available selection between for example thechannelized receivers, Bragg cell receivers, compressive receivers (and)digital FFT receivers identified in the Cohen patent document. Thepossibility that such a monobit receiver can be realized on a singleintegrated circuit chip especially makes a direction finder of thepresent invention type a realistic possibility and moreover greatlyenhances the prospect of this apparatus being sufficiently small andlight in weight as to enable its use in even a small military aircraft.Such a direction finder is the area of interest in the presentinvention. The direction finder of the present invention can of coursealso be used in other settings including use in connection with anunattended ground sensor or an unmanned air vehicle.

SUMMARY OF THE INVENTION

The present invention provides a simplified, small size, passive,instantaneous- operation, microwave direction finding and microwavesignal frequency identification system.

It is therefore an object of the present invention to provide asimplified, small size, passive, instantaneous-operation, microwavedirection finding and microwave signal frequency identification systemthat is based on a simplified unit value related approximation of theFourier transformation Kernel function$^{\frac{{- {j2}}\quad \pi \quad {kn}}{N}}.$

It is another object of the invention to provide a microwave directionfinding and microwave signal frequency identification system that iscompatible with use in a tactical military aircraft.

It is another object of the invention to provide a microwave directionfinding and microwave signal frequency identification system that may beof selected accuracy and complexity.

It is another object of the invention to provide a microwave directionfinding and signal frequency identification system in which the numberof Fourier transformation receivers may be selected.

It is another object of the invention to provide a microwave directionfinding and signal frequency identification system in which the numberof receiving antenna elements and the system resolution capability maybe selected.

These and other objects of the invention will become apparent as thedescription of the representative embodiments proceeds.

It is another object of the invention to provide a microwave directionfinding and signal frequency identification system that may be used inboth military and civilian endeavors.

These and other objects of the invention are achieved by the passivemethod of identifying both operating frequency and relative angularlocation of a distant source of microwave radio frequency radiant energywith respect to a receiving location, said method comprising the stepsof:

receiving, in multiple elements of a circular disposed omni directionalmicrowave antenna located in said receiving location, multiple antennaelement samples of energy radiated from said distant source of microwaveradio frequency radiant energy;

coupling electrical signals, generated by said received radiated energyin each of said circular disposed omni directional microwave antennaelements, through a mode forming electrical matrix to phase segregatedmultiple output ports of said mode forming electrical matrix;

communicating each of said mode forming electrical matrix phasesegregated multiple output port electrical signals to a separate monobitelectronic warfare radio receiver of substantially unit value Fouriertransformation Kernel function$^{\frac{{- {j2}}\quad \pi \quad {kn}}{N}}$

realization and signal phase angle preserving characterization;

determining, from Fourier transformation of each said communicated phasesegregated multiple output port electrical signal in one of said monobitelectronic warfare radio receivers, a predominant signal frequencycomponent of said energy radiated from said distant source of microwaveradio frequency radiant energy;

ascertaining, from phase decoding of Fourier transformations of multipleof said communicated, phase segregated, multiple output port electricalsignals from said electronic warfare radio receivers, an angle ofarrival vector, with respect to said receiver location, for said energyradiated from said distant source of microwave radio frequency radiantenergy.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings incorporated in and forming a part of thespecification, illustrate several aspects of the present invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 shows a military scene possibly involving the present invention.

FIG. 2 shows details of a present invention direction finding and signalfrequency- determining system usable in the FIG. 1 scene.

FIG. 3 shows additional details of a Butler Matrix usable in the FIG. 2system.

FIG. 4 shows Kernel function approximations usable in the FIG. 2 system.

FIG. 5 shows a circular microwave antenna of a type usable in the FIG. 2system.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 in the drawings shows a military scene in which a directionfinding and signal frequency-determining system according to the presentinvention may be used. In the FIG. 1 scene a tactical military aircraft100 has ventured into the operating range of a ground based radar system101 located in the building 102 and operating by way of anelectronically steerable antenna array 104. The radar apparatus 101 maybe of a long-range search nature or of a shorter-range weapons directingtype. The radar system 101 is transmitting and receiving pulses ofmicrowave radio frequency energy along a path 106 between the antenna104 and the aircraft 100. A portion of the energy transmitted fromantenna 104 is reflected by the aircraft 100 back to the antenna 104 andprovides the signal by which the radar system 101 is viewing theaircraft 100.

Of interest with respect to the present invention, another portion ofthis FIG. 1 transmitted radar energy is received in acircular-configured microwave antenna 110 housed within a radome 108both of which are disposed on a suitable external portion of theaircraft 100. By way of this antenna 110-received microwave radiofrequency energy, it is desirable for the crew of the aircraft 100 to beappraised of both the occurrence of the FIG. 1 represented radar lock-onand also be informed of the operating frequency and possibly otheroperating details concerning the radar system 101. Such informing is ofcourse useful in confirming that the radar system 101 is indeed nonfriendly for example and can also alert the aircraft crew as to thesearching or tracking nature of the radar and thus of the immediatepossibility of incoming threat weapons. Preferably such appraisal isformulated within the duration of the first pulse of radio frequencyenergy received from the radar system 101 or in response to this firstpulse of energy or at least within a short system-delayed response tothis first pulse. This is especially important if the threat signal isof short duration, is difficult to intercept or if the threat system ismoving. The radar system 101 may also represent a system mounted on avehicle including another aircraft, a system which again may be of asearch or a tracking nature.

The aircraft-mounted antenna represented at 110 in FIG. 1 is preferablyof an omni directional and multiple signal elevation angle receptiontype in order to assuredly and efficiently receive incoming signals fromany possible location around the aircraft 100. Additional details of anantenna suitable for use in the location 110 are disclosed in theparagraphs following and especially in connection with the FIG. 5drawing herein. One aspect of the desired antenna is that it becomprised of a plurality of elements each having a principle boresightaxis extending in small angular azimuth disposition with respect to thesimilar axis of adjacent elements. This directivity characteristic maybe supplemented with steering or beamforming action.

FIG. 2 In the drawings shows the antenna 110 of the FIG. 1 aircraft 100together with a block diagram of a direction finding and signalfrequency-determining system according to the present invention. In theFIG. 2 drawing the antenna 110 is connected by way of a plurality oftransmission line elements 200, such as coaxial transmission lines, withthe first block 202 of the FIG. 2 system. Each element of the antenna110 connects with a different one of the transmission lines 200 and eachtransmission line 200 connects with a separate input node of the block202. The FIG. 1 and FIG. 2 antenna and its corresponding structure inthe FIG. 5 drawing may be described as having a sunflower petal-likearrangement of sensing elements that are disposed on an electricallyinsulating substrate and applied to a surface portion of the aircraft100.

The use of an electrical network or an electrical matrix, as is embodiedin the block 200 in FIG. 2, to couple signals between the elements of abeamforming antenna array and a signal generating or signal usingapparatus is now often practiced in the radio frequency electronic art.One of the most desirable network arrangements for performing thisbeamforming signal coupling function is known by the name of a “ButlerMatrix” i.e., an array of interconnected microwave radio frequencysignal processing elements usually inclusive of power dividers, phaseshifters and hybrids of plural varieties. A Butler Matrix is oftenarranged to have a differing number of signal input and signal outputports one number of ports being equal to the number of antenna elementsand the other number being equal to the number of ports of thetransmitting or receiving apparatus coupled to the system antenna. Suchelectrical networks or matrices are also identified as modeformers andmay be said to mathematically relate antenna signals and electrical modesignals according to a selected mathematical relationship, i.e., acomplex mathematical matrix. By way of an electrical network, or amatrix such as the Butler Matrix, the output energy of a radio frequencytransmitter for example may be divided into phase related portionssuitable for energizing the different elements of a multiple elementantenna array to produce for example a beam of radiated energy directedin one specific azimuth and elevation-defined direction with respect tothe antenna array. An opposite similar function is performed in the caseof a radio receiver system using a Butler Matrix with signals from eachazimuth direction around the antenna being converted to electricalwaveforms of a unique phase relationship.

A similar function, of perhaps more relevance in the present directionfinding invention setting, is performed by such a matrix during energyreception by the antenna system with signals from each azimuth directionaround the system being converted to electrical waveforms of a uniquephase relationship. Thus energy received by multiple elements in theFIG. 2 antenna array 110 is so combined in phase and amplitude at theoutput ports of the matrix 202 that identification of the direction ofarrival of the received energy with respect to the antenna arrayelements is possible. An early description of the Butler Matrixpreferred for this use is found in the published article “Beam FormingMatrix Simplifies Design of Electronically Scanned Antennas” authored byJ. Butler and R. Lowe and said to appear in the journal “ElectronicDesign” volume 9, pages 170-173, 1961. Another published articleconcerning the Butler Matrix is titled “Multiple Beam on Linear Arrays”authored by J.P. Shelton and K.S. Kelleher and appearing in theInstitute of Radio Engineers, Transactions on Antennas and Propagation,March 1961.

Additional descriptive material concerning the Butler Matrix is to befound in a number of U.S. patents including the U.S. Pat. No. 3,255,450patent of J.L. Butler, the U.S. Pat. No. 3,517,309 patent of C.W. Gerstet al., the U.S. Pat. No. 3,731,217 patent of C.W. Gerst et al., theU.S. Pat. No. 4,231,040 patent of S.H. Walker, the U.S. Pat. No.4,424,500 patent of R.D. Viola et al., the U.S. Pat. No. 5,373,299patent of E.T. Ozaki et al., and the U.S. Pat. No. 5,691,728 patent ofA.C. Goetz et al. Schematic drawings and related text concerning aButler Matrix appear in the above-identified Tsui text at page 108 and109 and in the above-identified Lipsky text commencing at page 132 andalso at page 169. Each of the patent publication and textbook referencesidentified herein is also hereby incorporated by reference herein. Aschematic drawing of a 32 element Butler Matrix and its connectedantenna additionally appears as FIG. 3 herein.

Continuing with discussion of the FIG. 2 direction finding and signalfrequency-determining system, the phase related signals developed in theButler Matrix 202 thus have differing phase relations according to thedirection of or the angle of arrival of the signals from the radarsystem 101 in FIG. 1. Each new Butler Matrix output signal at 204 infact comprises a signal representing the angular relationship betweenthe aircraft 100 and the radar system 101. The data on the referencepath 205 is for example of a coarse angular relationship nature and maybe viewed as being one angular signal manifestation appearing in a nosignal background while the data on the path 207 represents the inputdata with double the resolution of the path 205 data and with two signalrepresentations against the background; i.e., with two ambiguities. In asimilar manner the signals on the paths 209, 211 and 213 represent inputdata with successively doubled degrees of resolution but doubled numberof ambiguities. By decoding these signals in combination, asaccomplished in the block 224 of FIG. 2, an accurate non-ambiguousindication of the angle of arrival of the signal along path 106 withrespect to the aircraft 100 is provided at the system output port 222.The number of signals employed at locations 204, 208 and 213 in thepresent invention may be selected and may of course differ from the fivesignals represented in the FIG. 2 drawing in simpler or more complexarrangements of the invention.

In the block 206 of FIG. 2 there is located a plurality of limitingamplifier circuits used to condition the phase related signals appearingat 204 on the output paths 207, 209, 211, 213 and 205 of the ButlerMatrix in block 202. These limiting amplifier circuits improve theaccuracy of the FIG. 2 system by increasing the amplitude of each signalat 204 to such degree that only the zero crossings or othermanifestations of signal phase remain discernable in the limitingamplifier output signal. Amplifier circuits that are driven intosaturation are commonly used for embodiment of limiting amplifiers asrepresented in block 206. Cost limited lower performance arrangements ofthe FIG. 2 system may possibly omit the amplifiers of block 206 with therealization that resulting angle of arrival determinations can be lessaccurate especially in the instance of weaker input signals.

The output signals of the limiting amplifiers of block 206 appearcollectively at 208 and are applied to the respective individual monobitreceivers 210, 212, 214, 216 and 218 in the FIG. 2 system embodiment.Fundamentally the monobit receivers 210, 212, 214, 216 and 218 serve todetermine the Fourier transformation or the frequency components of eachsignal applied along the paths at 208. These frequency componentsclearly identify the signal being received by the FIG. 2 system by itscomponent parts and thereby implement the signal identification functiondesired from the system. For search speed enhancement and superiorfrequency resolution it is desired that each monobit receiver 210, 212,214, 216 and 218 have as many frequency channels as are needed to coverthe desired bandwidth of the system, a bandwidth of about 1 gigahertzbeing desirable for the overall FIG. 2 system. Individual channels inthe receivers 210, 212, 214, 216 and 218 are desirably rather narrow, ofabout 10 megahertz bandwidth, in order to segregate signals separated bymore than 10 megahertz in the provided signal identification. Otherbandwidths and resolutions are however possible.

As indicated earlier herein the radio receivers 210, 212, 214, 216 and218 may be embodied in the form of several possible receiver typeshowever for the present airborne and reasonable cost and complexitysystem the use of the monobit microwave receiver (MBR) described in theidentified patents originating in our same laboratory and involvinginventor J.B.Y. Tsui is considered preferable. This receiver employs theunit value approximated Kernel function in a discrete Fouriertransformation realization and is thereby of considerably reducedcomplexity and physical size with respect to the other receivers usableat 210, 212, 214, 216 and 218 in the FIG. 2 system. The monobit receiverhas somewhat limited two tone instantaneous dynamic range, acharacteristic resulting in the FIG. 2 system processing only thestronger of two signals that are separated by more than five dB ofsignal strength. If two incoming signals are of signal strength withinfive dB of each other the monobit receiver can report one or morefrequencies correctly and under most simultaneous signal conditions doesnot generate erroneous frequency information as other receivers do. Thesimultaneous signal condition is found to be better resolved by themonobit receiver of the present invention than by other possiblereceivers.

Since the monobit receiver is based on the discrete Fouriertransformation the received phase relationships are maintained in thereceivers 210, 212, 214, 216 and 218 and the discrete Fouriertransformation outputs are complex quantities. In these output signalsphase relationships may be determined from the relationship:

φ=[Imaginary(X(k))/Real(X(k))]  (1)

where X(k) is the kth frequency component.

In the FIG. 2 described system each of the five illustrated phasechannels is preferably arranged to be capable of reporting one hundredfrequency outputs. As shown in the FIG. 6 with a representative inputsignal an output signal appears at the same frequency channel for eachof the five receivers at 210, 212, 214, 216 and 218 in the FIG. 2system. The vertical scales in the FIG. 6 drawings represent signalamplitude, i.e., an amplitude that may be determined from therelationship:

Amplitude={[Imaginary(X(k))]²+[Real(X(k))]²}^(1/2)   (2)

The equation 2 relationship indicates only the location of a signalbecause in the present invention the input is. hard limited by thelimiting amplifiers shown at 206 in FIG. 2 and therefore does notprovide accurate amplitude information. In other words changes inreceiver input signal magnitude are not accurately reflected at theoutput of the limiting amplifiers 206 in view of such limiting action.Nevertheless however such an input signal does generate an output signalhaving real and imaginary components which sum in the manner indicatedby equation 2 to provide some form of an output signal. The maintenanceof phase relationship in the monobit receiver however enables thecomparison of phase among the five phase channels to produce the desiredangle of arrival data from the multiple simultaneous signals. Thedescribed system therefore provides frequency and angle of arrivalinformation from multiple simultaneous signals. In the system, frequencyinformation is obtained through the monobit receivers and angle ofarrival information is obtained from the combination of the Butlermatrix and the circular antenna array.

At 224 in the FIG. 2 system is shown the encoding logical circuitryserving to obtain at 222 one angle of arrival value from the fivemonobit receiver phase output signals at 213. This encoding logicperforms the phase angle to angle of arrival conversion function usingthe signal from the fifth monobit receiver 218 as a reference signal.The output of each other monobit receiver is compared to this referencesignal to obtain the described signals of differing resolution anddegrees of ambiguity. According to this arrangement each monobitreceiver measures the same signal and reports it frequency. By usingmultiple receivers and comparing their outputs to the reference themultiple results are used to determine what is ambiguous and select thenon-ambiguous.

The signal paths 219 and 220 and the video amplifier 217 in the FIG. 2system provide a conveyance by which a video signal from one ButlerMatrix output port reaches the encoding logic of block 224. The encodinglogic of block 224 includes both frequency discrimination and selectionlogic to identify signals of interest. The encoding logic 224 alsoincludes a phase comparison function that is preferably disposed insoftware form. Threshold logic also in the encoding logic of block 224can additionally determine the frequencies and direction of othersimultaneous signals within a given margin such as ten megahertzresolution; such resolution depends on tradeoffs and system needsincluding sampling speed, memory, hardware size, discrete Fouriertransform length, logic complexity and hardware size.

FIG. 4 in the drawings shows the Kernel function locations preferred foruse in the approximated Fourier transformation of the present invention.The FIG. 4 drawing originates as FIG. 2 in one of the above identifiedand incorporated by reference herein previous U.S. patent applicationsinvolving inventor J. B. Y. Tsui and colleagues, the application of Ser.No. 09/944,616. In the FIG. 4 drawing four Kernel function values ofprecisely unit magnitude length appear at 408, 410, 412 and 414 and fourKernel function values of actually 1.414 magnitude, a magnitude that canbe successfully regarded as also having unit length are shown at 400,402, 404 and 406. As is disclosed in the Ser. No. 09/944,616 applicationthe combination of these FIG. 4 eight Kernel function values is found toprovide an approximate Kernel function realization achieving increasedinstantaneous dynamic range and possibly other benefits with respect toa monobit receiver using either of two previously disclosed four valueKernel function approximations. The present invention is of course notlimited to the FIG. 4 Kernel function values and may be used with eitherof the four unit value Kernel function approximations or otherapproximations. In the present document the various approximation Kernelfunction values of unity or near unity magnitude are referred-to ashaving substantially unit value.

The arrangement shown in FIG. 5 is representative of an antenna systemusable with the present invention. The FIG. 5 antenna system 500 may befabricated on an insulating substrate material, as appears at 512 and514 in FIG. 5; such materials as the plastic- impregnated woven cloth orother electrical insulating sheet stock including the fiberglass duroidmaterial, may be used. Antennas of this type are also available fromsuppliers such as Anaren Microwave Corporation of New York, USA. TheFIG. 5 antenna system includes a total of thirty-two individual antennasor elements as are represented by the typical elements 502 and 504. TheFIG. 5 system is shown slightly enlarged, is actually of some three andthirteen sixteenths inches or nine and seven tenths centimeters overalldiameter and is comprised of individual elements of seven eighths of aninch or two and two tenths centimeters length as are disposed annularlyin angular separations of 360/32 or 11.25 degrees; other separationssuch as ten degrees are also possible depending on the size, number andplacement of the elements.

The FIG. 5 antenna system is feasible for use with a direction findingarrangement of the present invention type, a system operating forexample in the frequency range of ten gigahertz. At the innermost end ofeach element of the FIG. 5 antenna system is disposed an impedancematching transformer having the appearance of a hole 506 received in thetypical copper antenna conductor material 508, a hole disposed betweenadjacent roots of the typical sunflower petal-like antenna elements 502and 504. Coaxial cable transmission lines connect to each element of theFIG. 5 antenna at the narrow air gap region 510 just external of thematching transformer holes 506. In this connection arrangement eachelement terminates its own coaxial cable center conductor and thesurrounding shield conductor of an adjacent element.

The foregoing description of the preferred embodiment of the presentinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive nor to limit theinvention to the precise form disclosed. Obvious modifications orvariations are possible in light of the above teachings. The embodimentwas chosen and described to provide illustration of the principles ofthe invention and its practical application and to thereby enable one ofordinary skill in the art to utilize the invention(s) in variousembodiments and with various modifications as are suited to the scope ofthe invention determined by the appended claims when interpreted inaccordance with the breadth to which they are fairly, legally andequitably entitled.

We claim:
 1. The real time passive digital method of identifying bothoperating frequency and relative angular location of a distant source ofmicrowave radio frequency radiant energy with respect to a receivinglocation, said method comprising the steps of: receiving, in multipleelements of a circular disposed omni directional microwave antennalocated in said receiving location, multiple antenna element samples ofenergy radiated from said distant source of microwave radio frequencyradiant energy; coupling electrical signals, generated by said receivedradiated energy in each of said circular disposed omni directionalmicrowave antenna elements, through a mode forming electrical matrix tophase segregated multiple output ports of said mode forming electricalmatrix; communicating each of said mode forming electrical matrix phasesegregated multiple output port electrical signals to a separate realtime monobit electronic warfare radio receiver of substantially unitvalue real time digital Fourier transformation Kernel function$^{\frac{{- {j2}}\quad \pi \quad {kn}}{N}}$

realization and signal phase angle preserving characterization;determining, from real time digital Fourier transformation of saidcommunicated phase segregated multiple output port electrical signal inone of said monobit electronic warfare radio receivers, a predominantsignal frequency component of said energy radiated from said distantsource of microwave radio frequency radiant energy; and ascertaining,from phase decoding of digital Fourier transformations of multiple ofsaid communicated, phase segregated, multiple output port electricalsignals in said electronic warfare radio receivers, an angle of arrivalvector, with respect to said receiver location, from said energyradiated from said distant source of microwave radio frequency radiantenergy.
 2. The real time passive digital method of identifying operatingfrequency and relative angular location of a distant source of microwaveradio frequency radiant energy of claim 1 wherein said receivinglocation is disposed on a moving aircraft and said distant source ofmicrowave radio frequency radiant energy comprises one of an airborneand a ground disposed radar system.
 3. The real time passive digitalmethod of identifying operating frequency and relative angular locationof a distant source of microwave radio frequency radiant energy of claim1 further including the step of dispersing said circular disposed omnidirectional microwave antenna elements into a flower petal-like array ofindividually signal output port-coupled elements.
 4. The real timepassive digital method of identifying operating frequency and relativeangular location of a distant source of microwave radio frequencyradiant energy of claim 1 wherein said coupling step is comprised ortransferring said electrical signals through a mode forming ButlerMatrix.
 5. The real time passive digital method of identifying operatingfrequency and relative angular location of a distant source of microwaveradio frequency radiant energy of claim 1 wherein said step ofcommunicating each of said mode forming electrical matrix phasesegregated multiple output port electrical signals to a separate monobitelectronic warfare radio receiver of near unit value real time digitalFourier transformation Kernel function$^{\frac{{- {j2}}\quad \pi \quad {kn}}{N}}$

includes a Kernel function of exactly unit value.
 6. The real timepassive digital method of identifying operating frequency and relativeangular location of a distant source of microwave radio frequencyradiant energy of claim 1 wherein said step of communicating each ofsaid mode forming electrical matrix phase segregated multiple outputport electrical signals to a separate monobit electronic warfare radioreceiver of near unit value real time digital Fourier transformationKernel function $^{\frac{{- {j2}}\quad \pi \quad {kn}}{N}}$

includes a Kernel function of magnitude equal to the square root of two.7. The real time passive digital method of identifying operatingfrequency and relative angular location of a distant source of microwaveradio frequency radiant energy of claim 1 wherein said determining stepfurther includes real time digital Fourier transformation of each saidcommunicated phase segregated multiple output port electrical signal ina separate one of said monobit electronic warfare radio receivers andmultiple determinations of a predominant signal frequency component ofsaid energy radiated from said distant source of microwave radiofrequency radiant energy.
 8. The real time passive digital method ofidentifying operating frequency and relative angular location of adistant source of microwave radio frequency radiant energy of claim 1further including the step of communicating signals from an output pathof said mode forming electrical matrix to a signal input path of anangle of arrival computation.
 9. The real time passive digital method ofidentifying operating frequency and relative angular location of adistant source of microwave radio frequency radiant energy of claim 1further including processing output signals of said mode formingelectrical matrix in a saturating amplitude limiting circuit.
 10. Thereal time passive digital method of identifying operating frequency andrelative angular location of a distant source of microwave radiofrequency radiant energy of claim 1 wherein said step of ascertaining anangle of arrival vector from phase decoding of digital Fouriertransformations of multiple of said communicated, phase segregated,multiple output port electrical signals includes decoding a plurality ofsaid phase segregated, multiple output port electrical signals each ofdifferent phase resolution and signal ambiguity characteristics. 11.Distant microwave signal source real time digital frequencyidentification and relative angular position-determination airbornetactical aircraft apparatus comprising the combination of: a circulardisposed multiple element microwave antenna member mounted in amicrowave energy signal reception location of a host tactical aircraft;a plurality of transmission line members each connecting between a feedpoint- location in one element of said multiple element microwaveantenna member and an antenna signal output port; a signal phaseresponsive beamforming electrical network having a plurality of signalphase processing elements disposed in an interconnected repetitiveelectrical matrix, said electrical matrix including an input portconnected with said antenna signal output port transmission lines and anoutput port having a lesser number of microwave energy signal phaserelated output paths; a plurality of Fourier transformation real timedigital microwave frequency radio receivers each of substantially unitvalue-realized $^{\frac{{- {j2}}\quad \pi \quad {kn}}{N}}$

Kernel function character, and having input port connection with one ofsaid signal phase responsive electrical network signal phase relatedoutput paths; said Fourier transformation real time digital microwavefrequency radio receivers each also having constant input and outputsignal phase relationships and having digital output signalsrepresentative of Fourier components comprising said distant source ofmicrowave radio frequency radiant energy operating frequencies; andsignal phase responsive angle of arrival determination apparatusconnected with each digital output signal of said Fourier transformationmicrowave frequency radio receiver and with an angle of arrival outputsignal port of said airborne apparatus.
 12. The distant microwave signalsource real time digital frequency identification and relative angularposition-determination tactical aircraft apparatus of claim 11 furtherincluding a plurality of signal amplitude limiter elements eachconnected intermediate one of said signal phase processing elements ofsaid signal phase responsive electrical network and an input path of oneof said Fourier transformation real-time digital microwave frequencyradio receivers.
 13. The distant microwave signal source real timedigital frequency identification and relative angularposition-determination airborne tactical aircraft apparatus of claim 11further including a signal amplifier having a signal input portconnected with one of said microwave energy signal phase related outputpaths of said repetitive matrix and a signal output port connected withan input port of said signal phase responsive angle of arrivaldetermination apparatus.
 14. The distant microwave signal source realtime digital frequency identification and relative angularposition-determination airborne apparatus of claim 11 wherein saidsignal phase responsive electrical network comprises a Butler Matrix.15. The distant microwave signal source real time digital frequencyidentification and relative angular position-determination airborneapparatus of claim 14 wherein said Butler Matrix includes a plurality ofmicrowave hybrid, microwave signal power divider and microwave signalphase shifter elements.
 16. The distant microwave signal source realtime digital frequency identification and relative angularposition-determination airborne apparatus of claim 14 wherein saidButler Matrix includes means for generating a plurality of phase relatedoutput signals each of differing and unit integer phase angle multiplerelationships.
 17. The distant microwave signal source real time digitalfrequency identification and relative angular position-determinationairborne apparatus of claim 14 further including a reference signal pathconnected between an output port of said Butler Matrix and a videoreference signal input node of said signal phase responsive angle ofarrival determination apparatus.
 18. The distant microwave signal sourcereal time digital frequency identification and relative angularposition-determination airborne apparatus of claim 11 wherein saidtransmission line members each comprise a length of coaxial cable. 19.The distant microwave signal source real time digital frequencyidentification and relative angular position-determination airborneapparatus of claim 11 wherein said circular disposed multiple elementmicrowave antenna member is comprised of circularly arrayed microwavefrequency antenna elements located in a sunflower petal-likerelationship on an electrical insulation substrate received in a surfaceportion of said tactical aircraft.
 20. The distant microwave signalsource real time digital frequency identification and relative angularposition-determination airborne apparatus of claim 11 wherein saidFourier transformation real time digital microwave frequency radioreceivers each include digital Fourier transformation apparatus of twohundred fifty-six data points and substantially one hundred nanosecondscycle time characteristics.
 21. The real time passive digital method ofidentifying both operating frequency and relative angular location of adistant source of microwave radio frequency radiant energy of claim 1wherein said step of communicating each of said mode forming electricalmatrix phase segregated multiple output port electrical signals to aseparate real time monobit electronic warfare radio receiver ofsubstantially unit value real time digital Fourier transformation Kernelfunction $^{\frac{{- {j2}}\quad \pi \quad {kn}}{N}}$

realization includes a two hundred fifty-six data points digital Fouriertransformation of substantially one hundred nanoseconds cycle time. 22.The real time passive digital method of identifying both operatingfrequency and relative angular location of a distant source of microwaveradio frequency radiant energy of claim 21 wherein said step ofcommunicating each of said mode forming electrical matrix phasesegregated multiple output port electrical signals to a separate realtime monobit electronic warfare radio receiver of substantially unitvalue real time digital Fourier transformation kernel function$^{\frac{{- {j2}}\quad \pi \quad {kn}}{N}}$

realization includes a two hundred fifty-six data points digital Fouriertransformation of one hundred two and four-tenths nanoseconds cycletime.